I don't believe US employees generate 3x the value of their European counterparts, but the justification is right there -- they're paid 3x (or whatever multiple) more.
The actual problem is how you measure "value". We're used to valuing technical work on technical merits, but the market just thinks in money, dollar/euro amounts. And of course it's no fault of European employees that their "value" is significantly lower than their US counterparts -- it's probably due to company leadership, differences in regulatory environment, geopolitical and macro-economical factors, and history (the fact that modern computing started in Silicon Valley is significant).
The actual problem is how you measure "value". We're used to valuing technical work on technical merits, but the market just thinks in money, dollar/euro amounts. And of course it's no fault of European employees that their "value" is significantly lower than their US counterparts -- it's probably due to company leadership, differences in regulatory environment, geopolitical and macro-economical factors, and history (the fact that modern computing started in Silicon Valley is significant).